Many types of electrical and electronic devices generate radiated and conductive interference signals. Radiated interference signals are typically broadcast through space. Conductive interference signals are typically conducted over power supply mains or power conductors. Radiated interference signals, if substantial enough in magnitude, may pose a health hazard. More typically however, conductive interference signals may adversely interfere with the proper operation of other electronic circuits which are located in close proximity and connected to the power supply mains. For these, and other reasons, electronic devices are subject to governmental and regulatory restrictions limiting the amount of interference which can be emitted from such products. Exemplary regulations include the Canadian "Limits and Methods of Measurement of Electromagnetic Disturbance Characteristics of Industrial, Scientific and Medical Radio Frequency Equipment," CAN/CSA-C108.6-M91(CISPR 11:1990), which were prepared by the Canadian Standards Association and approved by the Standards Council of Canada. The acronym "CISPR" refers to the Comite International Special des Perturbations Radioelectriques, also known as the International Special Committee on Radio Interference. The CISPR is the international committee that promotes unification by recommending approved standards to National Committees for adoption. Typically, the National Committees adopt the CISPR recommendations as their national rules in so far as national conditions will permit. Thus, exemplary regulations that set limits for interference characteristics of electrical lighting and similar equipment may be found in the CISPR reports.
One source of conductive interference in power control situations--including the situation where electrical lighting constitutes the load--is a dimmer which switches the applied current on and off during each half cycle of AC power to regulate the power delivered to the load and therefore the output of the load, e.g., the intensity of the light. Typical dimmers use silicon controlled rectifiers (SCRs) or triacs, both of which are hereinafter generically referred to as thyristors. The thyristor is usually mounted in a wall switch box or otherwise integrated with the lighting device itself. An example of a thyristor-based dimmer is described in U.S. Pat. No. Re. 35,220 which is assigned to the assignee hereof.
A thyristor generates interference signals as a result of an essentially instantaneous and virtually discontinuous current transition when switching from an off or non-conductive condition to an on or conductive condition, when a significant voltage exists across the thyristor at the time that the switching occurs. The instantaneous and discontinuous current transition is an inherent result of the switching action of the thyristor. The magnitude of the interference signal depends directly upon the magnitude of the current change with respect to time (di/dt). A relatively low di/dt value associated with the transition creates relatively low levels of interference. By comparison, larger di/dt values produce larger levels of interference signals.
Attempts to control the interference signals generated by thyristor-based dimmers have involved the addition of auxiliary attenuating circuit elements. The attenuating circuit elements have taken the form of filters which may be as simple as a capacitor or inductor, or as complex as an elaborate multi-pole, multi-component, complex filter using both passive and active elements. U.S. Pat. Nos. 5,264,761; 5,504,394 and 5,504,395, all assigned to the assignee hereof, describe examples of such filtering and attenuation devices used with a thyristor. These auxiliary attenuating elements add to the complexity and the manufacturing expense associated with the products in which the thyristors are employed.
The other known method of reducing interference involves the use of a transistor-based switching circuit which provides a slower and smoother switching transition. The transistor-based switching circuit can be slowly turned on since the conduction characteristics of a power control transistor are related to the bias signal of the transistor. Slowly increasing this bias signal using simple circuit elements causes the transistor to slowly switch into full conduction. Moreover, during the transition period, current is conducted through the transistor an amount that is proportional to the bias signal. Thus, the current change rate with respect to time (di/dt) through the transistor can be lowered based upon the simple circuit elements used to produce a gradual change in the bias signal. Since the magnitude of the interference signals is directly related to the magnitude of the di/dt, decreasing the di/dt reduces the magnitude of any generated interference signals. Typically, because of the lower di/dt, the transistor-based switching circuit avoids generating significant interference signals and is thus capable of complying with the pertinent regulations.
Even though the transistor-based circuits provide the desirable slow turn-on characteristics and thus achieve interference signal attenuation, they consume significant amounts of power. The forward conduction voltage of the power transistor and the forward bias voltages of rectification diodes required for use with the transistors result in power consumption. Because the power transistors are direct current devices, a diode bridge is required to rectify the AC current for use by the power transistor. For example, the voltage across the fully-conductive power transistors and diode bridge may be approximately 4-6 volts. The resulting power consumption may be high in high-current situations, for example, in the neighborhood of 60 or more watts in some lighting situations.
The power consumption translates into more heat generated by the switching circuit. Since high levels of heat will destroy the semiconductor elements, heat sinks are usually required to dissipate the heat. Heat sinks are relatively large and the addition of heat sinks to the circuit increases its overall size. The resulting size may be too large to integrate such a switching circuit into small spaces such as lamp sockets and lamp bases. Furthermore, the useful longevity of the semiconductor elements is decreased in a high-heat environment.
The thyristor-based dimmers, on the other hand, do not consume large amounts of power nor do they generate significant amounts of heat. Since the thyristor, once triggered into conduction, is the primary circuit element conducting current and thus consuming power, the power consumed by the circuit is dependent on the forward voltage characteristics of the thyristor. Typically, the voltage drop across the thyristor is approximately 1.2-1.6 volts. Thus, the power consumption is substantially reduced compared to the power consumption of a transistor-based switching circuit. Unfortunately, the di/dt values associated with known thyristor-based dimmers are large, as noted above, thus generating unacceptable levels of interference signals.
It is with respect to these and other factors that the present invention has evolved.